104 PHYCOMYCETEAE 



portions of the tubular thallus. Because of its diplanetism Sparrow (1943) 

 places this genus in the family Ectrogellaceae in the Saprolegniales. 

 Ectrogella, also parasitic in Diatoms, consists of short or somewhat 

 elongated, unbranched coenocytic tubes which become zoosporangia 

 whose zoospores are diplanetic, usually with the primary zoospores 

 biflagellate and encysting at the mouth of the exit tube or near by. 

 Sexual reproduction by the union of two adjacent thalli in the same host 

 cell has been reported. This results in the formation of a rounded, thick- 

 walled oospore lying more or less loosely in the oogone to which the 

 empty male cell remains attached by the persistent conjugation tube. 

 Eurychasma and Eurychasmidium, occurring in marine algae, have been 

 assigned by Sparrow to the same family. 



Family Thraustochytriaceae. Sparrow (1943) places the genus 

 Thraustochytrium in a family of its own. This fungus is Rhizophydium-\\ke, 

 growing saprophytically upon marine algae. There is a system of branched 

 rhizoids within the host cell and an external obpyriform zoosporangium 

 which renews itself by proliferation upon the discharge of the zoospores. 

 The latter are angular and not motile at first and later become trans- 

 formed into pyriform, biflagellate zoospores. Because of this similarity to 

 diplanetism Sparrow places this family in the Saprolegniales. 



Order Saprolegniales. This order consists of fungi with well-marked 

 hyphal development. In fact the largest fungus hyphae known are to be 

 found here. Thus Monsma (1937) found, growing on hemp seed in water, 

 hyphae of Achlya oblongata de Bary var. globosa Humphrey that attained 

 a diameter of 270 m near the base and were so stiff that, on removing the 

 seed from the water, the hyphae stood out straight to a length of 15 mm. 

 The fungus body in this order consists mostly of a well-developed system 

 of branching, usually tapering, filaments within the substratum and an 

 external portion mostly of coarse or slender, unbranched or less strongly 

 branched hyphae which bear the reproductive organs. These hyphae may 

 be nearly uniform in diameter or tapering gradually from base toward 

 the apex or they may be more or less regularly constricted here and there, 

 sometimes the orifices being closed by plugs of cellulin. In one family the 

 basal portion of the external mycelium consists of more or less rounded 

 or clavate bodies from whose upper portion arise slender, usually con- 

 stricted, hyphae. 



The mycelium of this order is coenocytic with a multinucleate layer 

 of cytoplasm surrounding a large central vacuole in the larger hyphae. 

 The cell walls show the cellulose reaction immediately upon treatment 

 with chloriodide of zinc. Usually septa are lacking in the mycelium 

 except: (1) to delimit zoosporangia, (2) to set apart the gametangia from 

 the remainder of the hyphae, and (3) to delimit injured portions. In 

 Achlya polyandra de Bary, Horn (1904) observed and showed to the 



